Many methods are currently in use which involve the generation of seismic waves at the surface of the earth and the recording of seismic waves at the surface after they have traveled downwardly into the earth, encountered subsurface anomalies of various kinds, and then have been deflected upwardly to the surface as by reflection, refraction, and diffraction. Numerous methods of generating the seismic waves are in use. So also are numerous methods for recording and processing the seismic waves. When these methods are used to determine the nature and structure of subsurface formations, they are called seismic surveying, and when the objective of the survey is to locate mineral deposits, the methods are sometimes referred to as seismic prospecting.
As currently practiced, seismic prospecting generally involves digitizing the received seismic waves and making magnetic tape recordings of digitized seismic waves and transporting the magnetic tape recordings to a computer center where they are processed in an effort to determine the nature and structure of subsurface formations.
Many different methods are employed for generating the seismic waves at or near the surface of the earth. They may be generated there in the form of a single sharp impulse, such as by detonation of an explosive at the bottom of a bore hole that has been drilled into the earth to a depth below the weathered layer. Sometimes the pulse is created by dropping a weight onto the surface of the earth, or by application of an impact to the surface of the earth, such as by pounding the earth with a piston or plate driven by a gas explosion. The point or area at which the seismic waves are generated is often called a shotpoint.
In another method of seismic prospecting as currently conducted, a train of seismic waves in the form of a "chirp" signal is generated in place of a sharp pulse, at the source.
A chirp signal is a frequency modulated wave of substantial duration in which the frequency is varied as a function of time, often a linear function, over a substantial period of as much as several seconds. The length of a chirp is to be contrasted with a duration of a single seismic impulse produced by the methods described above, wherein the total duration of the signal emitted from the source is only a small fraction of a second, such as about 0.05 sec. or less. Sometimes a chirp signal is referred to as a "sweep" since the frequency is swept from one value to another during such action.
After deflection by subsurface formations, the waves are detected at points at or near the surface of the earth by means of electromechanical transducers, sometimes called geophones or seismometers. Sometimes they are detected by means of an array of interconnected geophones arranged in predetermined relation to a geophone station. The combined waves from these transducers are often said to be received at the geophone station. Conventionally, the geophones convert the received seismic wave into a corresponding electrical voltage of analog form.
It has been common practice to transmit the seismic wave voltages developed at a set of geophone stations over a corresponding set of conductors to a recording system in the truck, or other recording vehicle. There the seismic wave voltages are multiplexed, then converted into digital signals, and then recorded in multiplexed form on a magnetic tape. The resultant record is called a digitized seismogram.
Such a prior art system generally requires the use of a separate pair of conductors for each geophone station. Thus where there are 24 geophone stations, 24 pairs of conductors are involved for transmitting the seismic wave signals to the recording vehicle.
When a chirp source signal is employed, the resultant recorded waves are cross-correlated with a replica of the original chirp signal in order to produce a record somewhat similar to that which would be produced if the original seismic wave had been in the form of an impulse. To achieve such a result, the received seismic waves are conventionally recorded on a magnetic tape along with a record of the chirp signal and the recorded seismic waves are cross-correlated with the record of the chirp signal at a computer center or in the truck. In order to increase the signal-to-noise ratio, a chirp signal may be separately produced 20 to 100 times at about the same shotpoint and received at the same receiver point and the resultant received waves subjected to equal-time compositing. The most commonly used seismic prospecting process involving the use of a chirp signal and the correlation of the chirp signal with the seismic records is known as "Vibroseis" (trademark of Continental Oil Company). For convenience, we will sometimes refer to such processes as a Vibroseis-type process.